A system was developed to culture principal cells from the proximal caput or distal caput epididymidis of the rat within a floating collagen matrix using serum-free medium. The cells retain their function for greater than 5 days. Using co-culture systems, we are studying the role of Sertoli cell secretions in maintenance of 5 alpha-reductase activity of culture principal cells. Active factors will be partially isolated from ovine rete testis fluid. The nature of protein secreted by cultured principal cells from several regions of the epididymis will be compared.